Downlink synchronization method, and apparatus and system cross-reference to related applications

ABSTRACT

Embodiments of the present disclosure provide a downlink synchronization method, an apparatus, and a system, and relate to the communications field, so as to reduce downlink synchronization complexity and improve downlink synchronization efficiency. According to the downlink synchronization method, abase station sends downlink synchronization reference information to UE by using dedicated signaling or a system message, where the downlink synchronization reference information is used to instruct the UE to perform downlink synchronization in a second cell by referring to a downlink synchronization channel of a first cell; and the base station sends, in the second cell according to location information of the UE, a beam including a downlink synchronization channel to the UE, where the first cell and the second cell include an overlapped coverage area. The embodiments of the present disclosure are applied to synchronization channel sending.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/984,221, filed on May 18, 2018, which is a continuation ofInternational Application No. PCT/CN2016/106298, filed on Nov. 17, 2016,which claims priority to Chinese Patent No. 201510819442.3, filed onNov. 20, 2015. All of the afore-mentioned patent applications are hereinincorporated by reference in their entireties.

TECHNICAL FIELD

Embodiments of the present application relate to the communicationsfield, and in particular, to a downlink synchronization method, anapparatus, and a system.

BACKGROUND

As a packet service and an intelligent terminal rapidly develop, ahigh-speed service with a large data amount has an increasingrequirement for a frequency spectrum. A millimeter wave is to become apotential target frequency spectrum for developing 5G (5th Generation)communications and 3GPP (3rd generation partnership project) Long TermEvolution Advanced (LTE-A) in the future. A centimeter wave frequencyband generally refers to a frequency spectrum ranging from 3 GHz to 30GHz, and a millimeter wave frequency band generally refers to afrequency spectrum ranging from 30 GHz to 300 GHz.

In the prior art, cellular communications such as LTE usually uses afrequency band of approximately 2 GHz or lower. An LTE-A small cellenhancement standardization project is studying and using a frequencyband of 3.5 GHz. In the IEEE (Institute of Electrical and ElectronicsEngineers) 802.11ad standard, a frequency band of 60 GHz is used for awireless local area network (WLAN), and is usually used for indoorcommunication at a short distance of approximately 10 meters. In theprior art, a frequency band of 6 GHz or higher has not been used in thecellular communications. A main challenge of using a millimeter-wavehigh frequency band in the cellular communications lies in that thiswave band faces relatively large free-space attenuation; in addition,extremely severe attenuation is caused by absorption and scattering fromair, rain, fog, buildings or other objects. A beamforming technology isconsidered as a potential technology that can compensate for a severemillimeter wave path loss. A massive multiple-input multiple-output(MIMO) system is considered as a potential direction for implementingthe beamforming technology on a millimeter-wave frequency band.

The IEEE 802.11ad standard supports beamforming. A process of beamtraining between two nodes in communication is as follows: A node 1separately sends training beacons in a plurality of different directionsby using a beam, and a node 2 receives the training beacons in aquasi-omni manner and identifies an optimal beam a; then the node 2separately sends beacons in a plurality of different directions by usinga beam, and the node 1 receives the beacons in a quasi-omni manner andidentifies an optimal beam b; and the node 2 reports the optimal beam ato the node 1, and the node 1 reports the optimal beam b to the node 2,so as to find an optimal matched beam pair. Subsequently, datacommunication is performed in directions of the beam pair. However, on alow frequency band in the cellular communications, a public signal of acell such as a synchronization channel or a broadcast channel is usuallytransmitted in an omni transmission manner rather than by using thebeamforming technology.

In LTE-A carrier aggregation (CA), larger bandwidth can be obtained byaggregating a plurality of contiguous or non-contiguous componentcarriers (CC), so that a peak data rate and a system throughput areincreased. CCs aggregated for UE are referred to as a serving cell. Theserving cell includes one primary cell (PCell) on a low frequency bandand at most four secondary cells (SCell) on a high frequency band. Theprimary cell is responsible for security of a non-access stratum (NAS),and the secondary cell mainly provides additional radio resources fordata communication. CA supports handover of the PCell and adding,deleting, activation, deactivation, and other operations on the SCell.

A downlink synchronization channel of the SCell may be sent in a shortdistance point-to-point communication manner in 802.11ad, or sentthrough omni transmission in the cellular communications. In addition,in the short distance point-to-point communication manner in 802.11ad, abase station and UE need to separately send training beacons to eachother in a plurality of different directions, so as to perform omni orquasi-omni beam training. A beam training process is relatively complex,and has a relatively large delay and relatively low system efficiency.Consequently, channel resources are wasted when the downlinksynchronization channel is transmitted in the omni transmission manner.When the millimeter-wave high frequency band is applied to a cellularcommunications system, the base station may use the beamformingtechnology to extend public channel coverage. However, in the prior art,a method for performing downlink synchronization in an SCell cannot beprovided when beamforming is directly applied to the cellularcommunications system.

SUMMARY

Embodiments of the present disclosure provide a downlink synchronizationmethod, an apparatus, and a system, so that downlink synchronization canbe implemented in a cellular communications system through beamforming.

According to a first aspect, a downlink synchronization method isprovided, including:

sending, by a base station, downlink synchronization referenceinformation to UE by using dedicated signaling or a system message,where the downlink synchronization reference information is used toinstruct the UE to perform downlink synchronization in a second cell byreferring to a downlink synchronization channel of a first cell; andsending, by the base station in the second cell according to locationinformation of the UE, a beam including a downlink synchronizationchannel to the UE, where the first cell and the second cell include anoverlapped coverage area.

According to the downlink synchronization method provided in thissolution, the base station sends the downlink synchronization referenceinformation to the UE by using the dedicated signaling or the systemmessage, and sends, in the second cell according to the locationinformation of the UE, the beam including a downlink synchronizationchannel to the UE, so that the UE detects, according to the receiveddownlink synchronization reference information, the downlinksynchronization channel in the beam sent by the base station. Thedownlink synchronization reference information can be used to instructthe UE to perform downlink synchronization in the second cell byreferring to the downlink synchronization channel of the first cell, sothat downlink synchronization is implemented in a cellularcommunications system through beamforming.

With reference to the first aspect, in a first possible implementation,before the sending, by the base station in the second cell, a downlinksynchronization channel to the UE according to location information ofthe UE, the method further includes:

determining, by the base station, that the UE needs to perform downlinksynchronization in the second cell.

Specifically, the determining, by the base station, that the UE needs toperform downlink synchronization in the second cell includes:

detecting, by the base station, a time length for sending the downlinksynchronization channel in the second cell to the UE; and

when determining that no downlink synchronization channel is sent in thesecond cell to the UE in a preset time length, determining that the UEneeds to perform downlink synchronization; or

determining, by the base station according to a status of detecting andreceiving a sounding reference signal SRS of the UE in the second cell,that the UE needs to perform downlink synchronization; or

determining, by the base station according to a bit error rate or ablock error rate during data transmission in the second cell with theUE, that the UE needs to perform downlink synchronization.

In this solution, the base station may actively trigger sending of thedownlink synchronization channel when determining that the UE needs toperform downlink synchronization in the second cell.

Alternatively, the determining, by the base station, that the UE needsto perform downlink synchronization in the second cell includes:

receiving, by the base station, a downlink synchronization request ofthe UE; and

determining, by the base station according to the downlinksynchronization request, that the UE needs to perform downlinksynchronization.

In this embodiment, when determining that the UE needs to performdownlink synchronization in the second cell, the UE may actively triggera request for asking the base station to send the downlinksynchronization channel in the second cell.

With reference to the first aspect or the first possible implementation,in a second possible implementation, before the sending, by a basestation, downlink synchronization reference information to UE by usingdedicated signaling or a system message, the method further includes:

configuring, by the base station, carrier aggregation for the UE, whereon an aggregated carrier, the first cell is on a carrier whose frequencydomain is located on a first frequency band, the second cell is on acarrier whose frequency domain is located on a second frequency band,the first frequency band is a relatively low frequency band, and thesecond frequency band is a relatively high frequency band.

With reference to anyone of the first aspect or the foregoing possibleimplementations, in a third possible implementation,

after the sending, by the base station in the second cell, a downlinksynchronization channel to the UE according to location information ofthe UE, the method further includes:

receiving a detection result that is of a downlink synchronizationchannel of the second cell and that is reported by the UE; anddetermining, by the base station according to the detection result,whether to configure the second cell as a serving cell of the UE.

In this solution, the base station can determine, according to thedetection result, whether to configure the second cell as the servingcell of the UE. Therefore, the base station may send a same downlinksynchronization reference signal to all UEs in the second cell, anddetermine, according to a detection result that is of the downlinksynchronization channel of the second cell and that is reported by UE,whether to configure the second cell as a serving cell of the UE. Aunified downlink synchronization reference signal is designed for allthe UEs, so that signaling design overheads are reduced.

With reference to anyone of the first aspect or the foregoing possibleimplementations, in a fourth possible implementation,

the sending, by the base station in the second cell, a downlinksynchronization channel to the UE according to location information ofthe UE includes:

sending, by the base station at any OFDM symbol location in a subframein the second cell according to the location information, the beamincluding a downlink synchronization channel to the UE.

Optionally, each OFDM symbol location is corresponding to one beam. AllUEs in a coverage area of one beam may receive a downlinksynchronization channel included in the beam. Two different beams aretwo beams whose maximum radiation directions do not overlap.

With reference to any one of the first aspect, or the first to the thirdpossible implementations, in a fifth possible implementation, thesending, by the base station in the second cell, a downlinksynchronization channel to the UE according to location information ofthe UE includes:

sending, by the base station at a fixed OFDM symbol location in anysubframe in the second cell according to the location information, thebeam including a downlink synchronization channel to the UE.

Optionally, each OFDM symbol location is corresponding to one beam. AllUEs in a coverage area of one beam may receive a downlinksynchronization channel included in the beam.

With reference to anyone of the first aspect or the foregoing possibleimplementations, in a sixth possible implementation,

each OFDM symbol includes beams in at least two directions, and all thebeams in the at least two directions use different radio frequencychains RF chains.

In addition, a fixed time offset is used between a time location of abeam of the downlink synchronization channel of the second cell and atime location of a beam of the downlink synchronization channel of thefirst cell. The downlink synchronization reference information mayfurther include but is not limited to at least one of the followinginformation: a mapping relationship between the time offset and a beamID of the downlink synchronization channel of the second cell, afrequency band of the second cell, bandwidth of the second cell, aphysical cell identifier (PCI) of the second cell, a frequency range inwhich the downlink synchronization channel of the second cell islocated, a quantity of beams of the second cell, or a beam width of thesecond cell.

According to a second aspect, a downlink synchronization method isprovided, including:

receiving, by UE, downlink synchronization reference information sent bya base station by using dedicated signaling or a system message, wherethe downlink synchronization reference information is used to instructthe UE to perform downlink synchronization in a second cell by referringto a downlink synchronization channel of a first cell; and

detecting, by the UE according to the downlink synchronization referenceinformation, a downlink synchronization channel of the second cell in abeam sent by the base station, where the first cell and the second cellincludes an overlapped coverage area.

According to the downlink synchronization method provided in thissolution, the base station sends the downlink synchronization referenceinformation to the UE by using the dedicated signaling or the systemmessage, and sends, in the second cell according to the locationinformation of the UE, the beam including a downlink synchronizationchannel to the UE, so that the UE detects the downlink synchronizationchannel according to the received downlink synchronization referenceinformation. The downlink synchronization reference information can beused to instruct the UE to perform downlink synchronization in thesecond cell by referring to the downlink synchronization channel of thefirst cell, so that downlink synchronization is implemented in acellular communications system through beamforming.

With reference to the second aspect, in a first possible implementation,

after the detecting, by the UE, a downlink synchronization channel ofthe second cell according to the downlink synchronization referenceinformation, the method further includes:

reporting, by the UE, a detection result of the downlink synchronizationchannel of the second cell to the base station.

In this solution, the base station can determine, according to thedetection result, whether to configure the second cell as the servingcell of the UE. Therefore, the base station may send a same downlinksynchronization reference signal to all UEs in the second cell, anddetermine, according to a detection result that is of the downlinksynchronization channel of the second cell and that is reported by eachUE, whether to configure the second cell as a serving cell of the UE. Aunified downlink synchronization reference signal is designed for allthe UEs, so that signaling design overheads are reduced.

With reference to the second aspect or the first possibleimplementation, in a second possible implementation, before thedetecting, by the UE, a downlink synchronization channel of the secondcell according to the downlink synchronization reference information,the method further includes:

determining, by the UE, that the UE needs to perform downlinksynchronization with the base station; and

sending, by the UE, a downlink synchronization request to the basestation.

In this solution, the base station may actively trigger sending of thedownlink synchronization channel when determining that the UE needs toperform downlink synchronization in the second cell.

With reference to the second possible implementation, in a thirdpossible implementation, the determining, by the UE, that the UE needsto perform downlink synchronization with the base station includes:

detecting, by the UE, a time length for receiving, in the second cell,the downlink synchronization channel sent by the base station; and

when determining that no downlink synchronization channel sent by thebase station is received in the second cell in a preset time length,determining, by the UE, that the UE needs to perform downlinksynchronization with the base station; or

determining, by the UE according to a status of detecting and receivingan SRS in the second cell, that the UE needs to perform downlinksynchronization with the base station; or

determining, by the UE according to a bit error rate or a block errorrate during data transmission with the base station in the second cell,that the UE needs to perform downlink synchronization with the basestation.

In this solution, when determining that the UE needs to perform downlinksynchronization in the second cell, the UE may actively trigger arequest for asking the base station to send the downlink synchronizationchannel in the second cell.

With reference to anyone of the second aspect or the possibleimplementations of the second aspect, in a fourth possibleimplementation,

the detecting, by the UE according to the downlink synchronizationreference information, a downlink synchronization channel of the secondcell in a beam sent by the base station includes:

detecting, by the UE according to information about the downlinksynchronization channel of the first cell, the downlink synchronizationchannel of the second cell in the beam sent by the base station.

According to a third aspect, a base station is provided, including:

a sending unit, configured to send downlink synchronization referenceinformation to UE by using dedicated signaling or a system message,where the downlink synchronization reference information is used toinstruct the UE to perform downlink synchronization in a second cell byreferring to a downlink synchronization channel of a first cell, where

the sending unit is further configured to send, in the second cellaccording to location information of the UE, a beam including a downlinksynchronization channel to the UE, where the first cell and the secondcell include an overlapped coverage area.

The base station provided in this solution sends the downlinksynchronization reference information to the UE by using the dedicatedsignaling or the system message, and sends, in the second cell accordingto the location information of the UE, the beam including a downlinksynchronization channel to the UE, so that the UE detects, according tothe received downlink synchronization reference information, thedownlink synchronization channel in the beam sent by the base station.The downlink synchronization reference information can be used toinstruct the UE to perform downlink synchronization in the second cellby referring to the downlink synchronization channel of the first cell,so that downlink synchronization is implemented in a cellularcommunications system through beamforming.

With reference to the third aspect, in a first possible implementation,the base station further includes:

a processing unit, configured to determine that the UE needs to performdownlink synchronization in the second cell.

Specifically, the processing unit is configured to detect a time lengthfor sending the downlink synchronization channel in the second cell tothe UE; and

when determining that no downlink synchronization channel is sent in thesecond cell to the UE in a preset time length, the processing unitdetermines that the UE needs to perform downlink synchronization; or

the processing unit is configured to determine, according to a status ofdetecting and receiving a sounding reference signal SRS of the UE in thesecond cell, that the UE needs to perform downlink synchronization; or

the processing unit is configured to determine, according to a bit errorrate or a block error rate during data transmission in the second cellwith the UE, that the UE needs to perform downlink synchronization.

In this solution, the base station may actively trigger sending of thedownlink synchronization channel when determining that the UE needs toperform downlink synchronization in the second cell.

Alternatively, the base station further includes a receiving unit,configured to receive a downlink synchronization request of the UE.

The processing unit is configured to determine, according to thedownlink synchronization request, that the UE needs to perform downlinksynchronization.

In this embodiment, when determining that the UE needs to performdownlink synchronization in the second cell, the UE may actively triggera request for asking the base station to send the downlinksynchronization channel in the second cell.

With reference to the third aspect or the first possible implementation,in a second possible implementation, the base station further includes:

a configuration unit, configured to configure carrier aggregation forthe UE, where on an aggregated carrier, the first cell is on a carrierwhose frequency domain is located on a first frequency band, the secondcell is on a carrier whose frequency domain is located on a secondfrequency band, the first frequency band is a relatively low frequencyband, and the second frequency band is a relatively high frequency band.

With reference to anyone of the third aspect or the foregoing possibleimplementations, in a third possible implementation,

the receiving unit is configured to receive a detection result that isof a downlink synchronization channel of the second cell and that isreported by the UE; and

the processing unit is configured to determine, according to thedetection result received by the receiving unit, whether to configurethe second cell as a serving cell of the UE.

In this solution, the base station can determine, according to thedetection result, whether to configure the second cell as the servingcell of the UE. Therefore, the base station may send a same downlinksynchronization reference signal to all UEs in the second cell, anddetermine, according to a detection result that is of the downlinksynchronization channel of the second cell and that is reported by eachUE, whether to configure the second cell as a serving cell of the UE. Aunified downlink synchronization reference signal is designed for allthe UEs, so that signaling design overheads are reduced.

With reference to anyone of the third aspect or the foregoing possibleimplementations, in a fourth possible implementation,

the sending unit is specifically configured to send, at different OFDMsymbol locations in a subframe in the second cell according to thelocation information, different beams including a downlinksynchronization channel to the UE.

With reference to any one of the third aspect, or the first to the thirdpossible implementations, in a fifth possible implementation, thesending unit is specifically configured to send, at a fixed OFDM symbollocation in each subframe in the second cell according to the locationinformation, different beams including a downlink synchronizationchannel to the UE.

With reference to anyone of the third aspect or the foregoing possibleimplementations, in a sixth possible implementation,

each OFDM symbol includes beams in at least two directions, and all thebeams in the at least two directions use different radio frequencychains RF chains.

In addition, a fixed time offset is used between a time location of abeam of the downlink synchronization channel of the second cell and atime location of a beam of the downlink synchronization channel of thefirst cell. The downlink synchronization reference information includesa mapping relationship between the time offset and a beam ID of thedownlink synchronization channel of the second cell. The downlinksynchronization reference information may further include but is notlimited to at least one of the following information: a frequency bandof the second cell, bandwidth of the second cell, a physical cellidentifier (PCI) of the second cell, a frequency range in which thedownlink synchronization channel of the second cell is located, aquantity of beams of the second cell, or a beam width of the secondcell.

According to a fourth aspect, UE is provided, including:

a receiving unit, configured to receive downlink synchronizationreference information sent by a base station by using dedicatedsignaling or a system message, where the downlink synchronizationreference information is used to instruct the UE to perform downlinksynchronization in a second cell by referring to a downlinksynchronization channel of a first cell; and

a detection unit, configured to detect, according to the downlinksynchronization reference information received by the receiving unit, adownlink synchronization channel of the second cell in a beam sent bythe base station, where the first cell and the second cell include anoverlapped coverage area.

In this solution, the base station sends the downlink synchronizationreference information to the UE by using the dedicated signaling or thesystem message, and sends, in the second cell according to locationinformation of the UE, the beam including a downlink synchronizationchannel to the UE, so that the UE detects the downlink synchronizationchannel according to the received downlink synchronization referenceinformation. The downlink synchronization reference information can beused to instruct the UE to perform downlink synchronization in thesecond cell by referring to the downlink synchronization channel of thefirst cell, so that downlink synchronization is implemented in acellular communications system through beamforming.

With reference to the fourth aspect, in a first possible implementation,the UE further includes:

a sending unit, configured to report a detection result of the downlinksynchronization channel of the second cell to the base station.

With reference to the fourth aspect or the first possibleimplementation, in a second possible implementation, the UE furtherincludes:

a processing unit, configured to determine that the UE needs to performdownlink synchronization with the base station; and

the sending unit is configured to send a downlink synchronizationrequest to the base station.

In this solution, when determining that the UE needs to perform downlinksynchronization in the second cell, the UE may actively trigger arequest for asking the base station to send the downlink synchronizationchannel in the second cell.

With reference to the second possible implementation, in a thirdpossible implementation, the processing unit is specifically configuredto detect a time length for receiving, in the second cell, the downlinksynchronization channel sent by the base station; and

when determining that no downlink synchronization channel sent by thebase station is received in the second cell in a preset time length, theprocessing unit determines that the UE needs to perform downlinksynchronization with the base station; or

the processing unit determines, according to a status of detecting andreceiving an SRS in the second cell, that the UE needs to performdownlink synchronization with the base station; or

the processing unit determines, according to a bit error rate or a blockerror rate during data transmission with the base station in the secondcell, that the UE needs to perform downlink synchronization with thebase station.

In this solution, when determining that the UE needs to perform downlinksynchronization in the second cell, the UE may actively trigger arequest for asking the base station to send the downlink synchronizationchannel in the second cell.

With reference to the fourth aspect, in a third possible implementation,the UE further includes:

a sending unit, configured to report a detection result of the downlinksynchronization channel to the base station.

With reference to anyone of the fourth aspect or the possibleimplementations of the fourth aspect, in a fourth possibleimplementation, the detection unit is specifically configured to detect,according to information about the downlink synchronization channel ofthe first cell, the downlink synchronization channel of the second cellin the beam sent by the base station.

According to a fifth aspect, a base station is provided, including aprocessor, a first interface circuit, a second interface circuit, amemory, and a bus. The processor, the first interface circuit, thesecond interface circuit and the memory are connected to each other andcommunicate with each other by using the bus. The processor isconfigured to execute a program in the memory, to perform the methodprovided in the first aspect or the possible implementations of thefirst aspect with reference to the first interface circuit and thesecond interface circuit.

Optionally, the processor may be configured to execute functions of theprocessing unit and the configuration unit in the third aspect or thepossible implementations of the third aspect.

The first interface circuit is configured to execute a function of thesending unit in the third aspect or the possible implementations of thethird aspect.

The second interface circuit is configured to execute a function of thereceiving unit in the third aspect or the possible implementations ofthe third aspect.

According to a sixth aspect, UE is provided, including a processor, afirst interface circuit, a second interface circuit, a memory, and abus. The processor, the first interface circuit, the second interfacecircuit, and the memory are connected to each other and communicate witheach other by using the bus.

The processor is configured to execute a program in the memory, toperform the method provided in the second aspect or the possibleimplementations of the second aspect with reference to the firstinterface circuit and the second interface circuit.

Optionally, the processor may be configured to execute functions of thedetection unit and the processing unit in the fourth aspect or thepossible implementations of the fourth aspect.

The first interface circuit is configured to execute a function of thereceiving unit in the fourth aspect or the possible implementations ofthe fourth aspect.

The second interface circuit is configured to execute a function of thesending unit in the fourth aspect or the possible implementations of thefourth aspect.

According to a seventh aspect, a communications system is provided,including any base station provided in the third aspect or the possibleimplementations of the third aspect and any UE provided in the fourthaspect or the possible implementations of the fourth aspect; or

including any base station provided in the fifth aspect and any UEprovided in the sixth aspect.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic structural diagram of a communications systemaccording to an embodiment of the present disclosure;

FIG. 2 is a schematic structural diagram of a communications systemaccording to another embodiment of the present disclosure;

FIG. 3 is a schematic flowchart of a downlink synchronization methodaccording to an embodiment of the present disclosure;

FIG. 4 is a schematic flowchart of a downlink synchronization methodaccording to another embodiment of the present disclosure;

FIG. 5 is a schematic flowchart of a downlink synchronization methodaccording to still another embodiment of the present disclosure;

FIG. 6 is a schematic flowchart of a downlink synchronization methodaccording to yet another embodiment of the present disclosure;

FIG. 7 is a schematic structural diagram of a frame structure accordingto an embodiment of the present disclosure;

FIG. 8 is a schematic structural diagram of a frame structure accordingto another embodiment of the present disclosure;

FIG. 9 is a schematic structural diagram of a frame structure accordingto still another embodiment of the present disclosure;

FIG. 10 is a schematic structural diagram of a base station according toan embodiment of the present disclosure;

FIG. 11 is a schematic structural diagram of UE according to anembodiment of the present disclosure;

FIG. 12 is a schematic structural diagram of a base station according toanother embodiment of the present disclosure; and

FIG. 13 is a schematic structural diagram of UE according to anotherembodiment of the present disclosure.

DESCRIPTION OF EMBODIMENTS

The following describes the technical solutions in the embodiments ofthe present disclosure with reference to the accompanying drawings inthe embodiments of the present disclosure.

Terms such as “component”, “module”, and “system” used in thisapplication are used to indicate computer-related entities. Thecomputer-related entities may be hardware, firmware, combinations ofhardware and software, software, or software in running. For example, acomponent may be, but is not limited to, a process that runs on aprocessor, a processor, an object, an executable file, a thread ofexecution, a program, and/or a computer. As an example, both a computingdevice and an application that runs on the computing device may becomponents. One or more components may reside within a process and/or athread of execution, and a component may be located on one computerand/or distributed between two or more computers. In addition, thesecomponents may be executed from various computer-readable media thathave various data structures. These components may communicate by usinga local and/or remote process and according to, for example, a signalhaving one or more data packets (for example, data from one component,where the component interacts with another component in a local systemor a distributed system, and/or interacts with other systems via anetwork such as the Internet by using a signal).

In addition, this application describes each aspect with reference to awireless network device. The wireless network device may be a basestation. The base station may be configured to communicate with one ormore user equipments, or may be configured to communicate with one ormore base stations having some functions of user equipment (for example,communication between a macro base station and a micro base station suchas an access point). Alternatively, the wireless network device may beuser equipment. The user equipment may be configured to communicate withone or more user equipments (for example, D2D communication), or may beconfigured to communicate with one or more base stations. The userequipment may be further referred to as a user terminal, and may includesome or all functions of a system, a subscriber unit, a subscriberstation, a mobile station, a mobile wireless terminal, a mobile device,a node, a device, a remote station, a remote terminal, a terminal, awireless communications device, a wireless communications apparatus, ora user agent. The user equipment may be a cellular phone, a cordlesstelephone set, a Session Initiation Protocol (SIP) phone, a smartphone,a wireless local loop (WLL) station, a personal digital assistant (PDA),a laptop computer, a handheld communications device, a handheldcomputing device, a satellite wireless device, a wireless modem card,and/or another processing device configured for communication in awireless system. The base station may be further referred to as anaccess point, a node, a node B, an evolved node B (eNB), or anothernetwork entity, and may include some or all functions of the foregoingnetwork entities. The base station may communicate with a wirelessterminal by using an air interface. The communication may be performedby using one or more sectors. The base station may convert a receivedair interface frame into an IP packet, and is used as a router betweenthe wireless terminal and a remaining part of an access network. Theaccess network includes an Internet Protocol (IP) network. The basestation may further coordinate air interface attribute management, andmay be further used as a gateway between a wired network and a wirelessnetwork.

All aspects, embodiments, or features are presented in this applicationby describing a system that may include multiple devices, components,modules, and the like. It should be appreciated and understood that,each system may include another device, component, module, and the like,and/or may not include all devices, components, modules, and the likediscussed with reference to the accompanying drawings. In addition, acombination of these solutions may be used.

In addition, the word “example” in the embodiments of the presentdisclosure is used to represent giving an example, an illustration, or adescription. Any embodiment or design scheme described as an “example”in this application should not be explained as being more preferred orhaving more advantages than another embodiment or design scheme.Exactly, “for example” is used to present a concept in a specificmanner.

In the embodiments of the present disclosure, information, signal,message, and channel may be interchangeably used sometimes. It should benoted that expressed meanings of the foregoing terms are consistent whendifferences are not emphasized. In addition, “of”, “corresponding,relevant”, and “corresponding” may be interchangeably used sometimes. Itshould be noted that expressed meanings of the foregoing terms areconsistent when differences are not emphasized.

Network architectures and service scenarios described in the embodimentsof the present disclosure are to describe the technical solutions in theembodiments of the present disclosure more clearly, but are not intendedto limit the technical solutions provided in the embodiments of thepresent disclosure. A person of ordinary skill in the art may understandthat, with evolution of the network architectures and appearance of anewservice scenario, the technical solutions provided in the embodiments ofthe present disclosure may still be applied to a similar technicalproblem.

The embodiments of the present disclosure may be applied to not only atime division duplex (TDD) scenario but also a frequency division duplex(FDD) scenario.

In the embodiments of the present disclosure, a 4G network scenario in awireless communications network is used for description. It should benoted that the solutions in the embodiments of the present disclosuremay be further applied to another wireless communications network, and acorresponding name may also be replaced with a name of a correspondingfunction in the another wireless communications network.

The embodiments of the present disclosure may be applied to an LTE-Acarrier aggregation technology. Carrier aggregation (CA) is performed ona carrier with a relatively low frequency band and a carrier with arelatively high frequency band (such as a millimeter wave and acentimetric wave) in the LTE-A technology, so as to provide largerbandwidth and capacity for user equipment UE. A carrier with arelatively low frequency band serves as a primary cell PCell. A carrierwith a relatively high frequency band serves as a secondary cell SCell.The PCell and the SCell are co-located. The PCell and the SCell arecovered by one base station or respectively covered by two differentbase stations. The SCell is located within a coverage area of the PCell,or coverage areas of the SCell and the PCell overlap. The relativelyhigh frequency band and the relatively low frequency band are relativeconcepts. That is, a frequency band used by the SCell is higher than afrequency band used by the PCell. The SCell is not necessarily limitedto a high frequency cell in the present disclosure, and this isapplicable to any scenario in which a synchronization channel is sent inthe SCell through beamforming. FIG. 1 is a schematic diagram ofcomposition of a communications system according to an embodiment of thepresent disclosure. An example in which a single eNB (evolved NodeB)communicates with user equipment (UE) is used for description in thepresent disclosure. This embodiment of the present disclosure alsosupports a scenario in which the UE communicates with the eNB by usingonly a high frequency cell rather than through CA. However, the UE mayobtain, by using a low frequency cell, information related to the highfrequency cell.

This embodiment of the present disclosure is also applicable to adual-connection scenario. This scenario includes backhaul communicationbetween a primary eNodeB (PeNB) of a PCell on a low frequency band and asecondary eNodeB (SeNB) of an SCell on a high frequency band andbackhaul communication between a primary secondary cell (PSCell) on alow frequency band and the secondary eNodeB of the SCell on a highfrequency band. As shown in FIG. 2, the PCell provides wide coverage andmobility management. The PSCell and the SCell mainly provide hotspotcoverage to increase a data communication throughput. In terms of asolution, a dual-connection scenario in FIG. 2 in which a PSCell and anSCell provided by an SeNB are combined is similar to a scenario in FIG.1 in which one eNB performs CA. Therefore, the system shown in FIG. 1 ismainly used as an example for description in the embodiments of thepresent disclosure. A beamforming technology related in each embodimentof the present disclosure may refer to a horizontal beam or a verticalbeam, or refer to a horizontal beam and a vertical beam.

Based on the communications system provided above, referring to FIG. 3,an embodiment of the present disclosure provides a downlinksynchronization method. The method includes the following steps.

101. Abase station sends downlink synchronization reference informationto UE by using dedicated signaling or a system message.

The downlink synchronization reference information is used to instructthe UE to perform downlink synchronization in a second cell by referringto a downlink synchronization channel of a first cell. The first celland the second cell include an overlapped coverage area. The dedicatedsignaling or system message may be sent by the base station by using thefirst cell or the second cell.

The UE and the base station are synchronous in downlink in the firstcell, and the UE and the base station are synchronous or nonsynchronousin downlink in the second cell. If the UE and the base station arenonsynchronous in downlink in the second cell, the base station sendsthe downlink synchronization reference information to the UE by usingdedicated signaling or a system message of the first cell.Alternatively, if the UE and the base station are synchronous indownlink in the second cell, the base station may send the downlinksynchronization reference information to the UE by using dedicatedsignaling or a system message of the first cell, or the base station maysend the downlink synchronization reference information to the UE byusing dedicated signaling or a system message of the second cell. Whenthe UE and the base station are synchronous in downlink in the secondcell, the base station or the UE may periodically trigger sending of thedownlink synchronization reference information, so as to ensure downlinksynchronous state of the UE in the second cell. Alternatively, whendetecting a deteriorated downlink synchronous state of the UE in thesecond cell, the base station or the UE triggers sending of the downlinksynchronization reference information.

A fixed time offset is used between a time location of a beam of adownlink synchronization channel of the second cell and a time locationof a beam of the downlink synchronization channel of the first cell. Thedownlink synchronization reference information may further include butis not limited to at least one of the following information: a mappingrelationship between the time offset and a beam ID of the downlinksynchronization channel of the second cell, a frequency band of thesecond cell, bandwidth of the second cell, a physical cell identifier(PCI) of the second cell, a frequency range in which the downlinksynchronization channel of the second cell is located, a quantity ofbeams of the second cell, or a beam width of the second cell.

102. The UE receives the downlink synchronization reference informationsent by the base station by using the dedicated signaling or the systemmessage.

103. The base station sends, in a second cell according to locationinformation of the UE, a beam including a downlink synchronizationchannel to the UE.

Before step 103, the base station may first obtain the locationinformation of the UE. The UE may report, in a previous process in whichthe UE interacts with the base station, the location information of theUE by using any cell that serves the UE. Optionally, the locationinformation may be beam index information.

Because a location of the UE can be obtained, when sending the downlinksynchronization channel in the second cell to the UE, the base stationmay configure, in preset time, the downlink synchronization channel onlyin a beam that points to the UE.

104. The UE detects, according to the downlink synchronization referenceinformation, a downlink synchronization channel of the second cell inthe beam sent by the base station.

Specifically, step 104 is as follows: The UE detects, according toinformation about the downlink synchronization channel of the firstcell, the downlink synchronization channel of the second cell in thebeam sent by the base station, for example, detects the downlinksynchronization channel of the second cell by referring to a frequencyresource in which the downlink synchronization channel of the first cellis located, for example, detection is performed near a center frequencyin which the downlink synchronization channel of the first cell islocated; and/or detects the downlink synchronization channel of thesecond cell by referring to a time domain resource in which the downlinksynchronization channel of the first cell is located, for example,detection is performed near a time at which the downlink channel of thefirst cell is sent. For detection on the downlink synchronizationchannel of the second cell, refer to the prior art. Details are notdescribed herein.

Further, during detection of the downlink synchronization channel in thesecond cell with reference to the downlink synchronization referenceinformation in this application, detection efficiency of the downlinksynchronization channel may be further improved. For example, a fixedtime offset is used between a time location of a beam of the downlinksynchronization channel of the second cell and a time location of a beamof the downlink synchronization channel of the first cell. The downlinksynchronization reference information includes a mapping relationshipbetween the time offset and a beam ID of the downlink synchronizationchannel of the second cell. After obtaining the mapping relationshipbetween the time offset and the beam ID of the downlink synchronizationchannel of the second cell, the UE may quickly detect the downlinksynchronization channel of the second cell according to the time offsetby referring to the downlink synchronization channel of the first cell.The beam ID is preconfigured for the UE or obtained each time the UEcommunicates with the base station by using a beam. The beam ID iscorresponding to a fixed time offset. Therefore, when receiving the beamsent by the base station, the UE may determine, according to the beamID, the time offset between the time location of the beam of thedownlink synchronization channel of the second cell and a time locationof a beam of the downlink synchronization channel of the first cell. Inaddition, the UE may quickly detect the downlink synchronization channelof the second cell by referring to the information about the downlinksynchronization channel of the first cell.

In addition, the UE may further detect the downlink synchronizationchannel on a frequency band and bandwidth of the second cell that areindicated by the base station, or detect the downlink synchronizationchannel in a frequency range in which the downlink synchronizationchannel of the second cell is located and that is indicated by the basestation.

In step 104, the UE may obtain, in a time window specified by the basestation or configured by the UE itself and by using the downlinksynchronization reference information obtained in step 102, a downlinksynchronization channel for synchronizing with the second cell. If nodownlink synchronization channel is received in a preset time window, orno downlink synchronization channel is detected in an SCell in thepreset time window, the UE reports error indication information to thebase station, and re-searches another cell and reports a measurementresult to the base station. The UE may trigger detection on a downlinksynchronization channel of a neighboring high frequency cell andmeasurement on the high frequency cell according to a measurement resultof a neighboring low frequency cell, and try to request the downlinksynchronization channel in the measured high frequency cell. The UE mayobtain time window information according to an identifier of a beam inwhich the UE is currently located and information about a time offsetbetween a downlink synchronization channel of the beam and a downlinksynchronization channel of a PCell.

After step 104, if the UE successfully detects the downlinksynchronization channel in the SCell, the UE may send indicationinformation to the base station at a preconfigured time-frequencyresource location, to notify the base station that the UE successfullyperforms downlink synchronization in the SCell.

According to the downlink synchronization method provided in thissolution, the base station sends the downlink synchronization referenceinformation to the UE by using the dedicated signaling or the systemmessage, and sends, in the second cell according to the locationinformation of the UE, the beam including a downlink synchronizationchannel to the UE, so that the UE detects, according to the receiveddownlink synchronization reference information, the downlinksynchronization channel in the beam sent by the base station. Thedownlink synchronization reference information can be used to instructthe UE to perform downlink synchronization in the second cell byreferring to the downlink synchronization channel of the first cell, sothat downlink synchronization is implemented in a cellularcommunications system through beamforming.

Specifically, for example, the first cell is a PCell, and the secondcell is an SCell. The base station may send the downlink synchronizationchannel to specified UE that needs to perform downlink synchronizationin the SCell. The following solution provides a downlink synchronizationmethod triggered by the base station. The base station actively triggerssending of the downlink synchronization channel when determining thatthe UE needs to perform downlink synchronization in the SCell. Referringto FIG. 4, the method includes the following steps.

201. The base station configures carrier aggregation for the UE.

On an aggregated carrier, the PCell is on a carrier whose frequencydomain is located on a first frequency band, and the SCell is on acarrier whose frequency domain is located on a second frequency band.The first frequency band is a relatively low frequency band, and thesecond frequency band is a relatively high frequency band. For example,the PCell is on a carrier whose frequency domain is located on a lowfrequency band (for example, less than 3 GHz), and the SCell is on acarrier whose frequency domain is located on a high frequency band (forexample, a centimetric wave frequency band or a millimeter wavefrequency band of 3.5 GHz, 5 GHz, over 6 GHz, or the like). The PCelluses a frame structure the same as that of the SCell, or the PCell usesa frame structure whose subframe length, symbol length, and subcarrierspacing are separately multiples of those of a frame structure of theSCell.

202. The base station sends downlink synchronization referenceinformation to the UE by using dedicated signaling or a system message.

203. The UE receives the downlink synchronization reference informationsent by the base station by using the system message of a PCell or thededicated signaling.

204. The base station determines that the UE needs to perform downlinksynchronization (has a downlink synchronization requirement).

In step 204, the base station may determine, in at least one of thefollowing manners or a combination of a plurality of the followingmanners, that the UE needs to perform downlink synchronization.

Manner 1: The base station detects a time length for sending thedownlink synchronization channel to the UE in the SCell; and

when determining that no downlink synchronization channel is sent to theUE in the SCell in a preset time length, the base station determinesthat the UE needs to perform downlink synchronization. An example isprovided for Manner 1: After sending the synchronization channel, basedon a timer or a time counter, the base station starts or restarts thetimer or resets the time counter to restart timing, so as to accordinglydetermine the preset time length.

Manner 2: The base station determines, according to a status ofdetecting and receiving a sounding reference signal (SRS) of the UE inthe SCell, that the UE needs to perform downlink synchronization. Forexample, if the base station determines that, at a preset orthogonalfrequency division multiplexing (OFDM) symbol location, the UE does notdetect a reference signal, or detects a time offset or a frequencyoffset between an OFDM symbol location of a reference signal and apre-estimated location, the base station determines that the UE needs toperform downlink synchronization.

Manner 3: The base station determines, according to a bit error rate ora block error rate during data transmission with the UE in the SCell,that the UE needs to perform downlink synchronization. For example, ifthe base station determines that the bit error rate or the block errorrate during data transmission with the UE in the SCell exceeds a presetthreshold, the base station determines that the UE needs to performdownlink synchronization with the base station.

Certainly, after the UE configures a new SCell or activates the SCell,the base station may actively trigger sending of the downlinksynchronization channel to the UE.

Manner 4: The base station determines, according to a received downlinksynchronization request message of the UE, that the UE needs to performdownlink synchronization. The UE may send the downlink synchronizationrequest message by using an uplink channel configured by the basestation. The uplink channel may be an uplink channel of the PCell.Certainly, if the UE is in an uplink synchronous state in the SCell, theuplink channel may be an uplink channel of the SCell.

205. When the base station determines that the UE needs to performdownlink synchronization in an SCell, the base station sends, accordingto location information of the UE, a beam including a downlinksynchronization channel to the UE in the SCell.

Before step 205, the base station may first obtain the locationinformation of the UE. For example, the base station periodicallyobtains the location information of the UE, or obtains locationinformation actively reported by the UE. For example, the UE may report,in a previous process in which the UE interacts with the base station,the location information of the UE by using any cell that serves the UE.The location information may be beam index information. The base stationmay determine a spatial location of the UE by using information of theUE in the PCell such as uplink information or channel state information(CSI), for example, determine the spatial location of the UE in theforegoing manner in a line of sight (LOS), so that a path loss of theuplink information or the CSI can be detected. A signal sendingdirection and a signal receiving direction can be obtained by the basestation, and a signal is attenuated after the signal is transmitted inthe air for a distance. Therefore, a fixed path loss may becorresponding to a distance of the UE relative to the base station.

Specifically, the base station may periodically send the downlinksynchronization channel in the PCell, for example, send a primarysynchronization channel (PSS) and a secondary synchronization channel(SSS) once each 5 ms according to an existing LTE technology. Todistinguish between two half-frames of a radio frame, different SSSs areusually sent in the first 5 ms and the last 5 ms of the radio frame. Forexample, the SSSs may be encoded differently. The PSS/SSS may spatiallycover the entire PCell, for example, cover a 120-degree sector area. Asequence of step 205 is not limited in this application. Step 205 may beperformed at any moment after the base station configures carrieraggregation for the UE, provided that step 205 is performed before theUE detects the downlink synchronization channel according to thedownlink synchronization reference information. An encoding format and achannel format of the downlink synchronization channel are not limitedin this application. For example, the downlink synchronization channelmay use a manner similar to that of combining two synchronizationchannels PSS and SSS in LTE, or the downlink synchronization channel maybe a single synchronization channel, and does not use a manner similarto that of combining two synchronization channels PSS and SSS in LTE.

Optionally, the base station may instruct, in advance by using the PCellor the SCell, the UE to prepare to receive the downlink synchronizationchannel in the SCell. The base station may instruct the UE to receiveinformation in the SCell such as a start time or a beam range of thedownlink synchronization channel. The base station sends, by using abeam, the downlink synchronization channel to the spatial location ofthe UE. The base station sends the downlink synchronization channel inthe SCell and the downlink synchronization channel in the PCellsynchronously in time domain. For example, a fixed time offsetrelationship exists in time domain between a specified beam (a specifiedbeam identifier may be indicated by using a beam ID) of sending adownlink synchronization channel in the SCell and a time at which adownlink synchronization channel is sent in the PCell. If the basestation cannot accurately determine the spatial location of the UE, thebase station may send, in a specific spatial area, a plurality of beamsincluding a downlink synchronization channel to the UE.

206. The UE detects, according to the downlink synchronization referenceinformation, a downlink synchronization channel of the SCell in the beamsent by the base station.

According to the downlink synchronization method provided in thisembodiment of the present disclosure, the base station sends thedownlink synchronization reference information to the UE by using thededicated signaling or the system message, and sends, in the second cellaccording to the location information of the UE, the beam including adownlink synchronization channel to the UE, so that the UE detects,according to the received downlink synchronization referenceinformation, the downlink synchronization channel in the beam sent bythe base station. The downlink synchronization reference information canbe used to instruct the UE to perform downlink synchronization in thesecond cell by referring to the downlink synchronization channel of thefirst cell, so that downlink synchronization is implemented in acellular communications system through beamforming. In addition, in thisembodiment, the base station may actively trigger sending of thedownlink synchronization channel when determining that the UE needs toperform downlink synchronization in the second cell.

Different from the embodiment shown in FIG. 4, in anotherimplementation, when UE determines that the UE needs to perform downlinksynchronization in an SCell, the UE may actively trigger a request forasking the base station to send a downlink synchronization channel inthe SCell. Referring to FIG. 5, another embodiment of the presentdisclosure provides a downlink synchronization method. The methodincludes the following steps.

301. The base station configures carrier aggregation for the UE.

On an aggregated carrier, a PCell is on a carrier whose frequency domainis located on a first frequency band, and the SCell is on a carrierwhose frequency domain is located on a second frequency band. The firstfrequency band is a relatively low frequency band, and the secondfrequency band is a relatively high frequency band. For a specificexample, refer to description in step 201. Details are not describedherein.

302. The base station sends downlink synchronization referenceinformation to the UE by using dedicated signaling or a system message.

303. The UE receives the downlink synchronization reference informationsent by the base station by using the dedicated signaling or the systemmessage of a PCell.

304. The UE determines that the UE needs to perform downlinksynchronization with the base station (has a downlink synchronizationrequirement).

Specifically, in step 304, the UE may determine, in at least one of thefollowing manners or a combination of a plurality of the followingmanners, that the UE needs to perform downlink synchronization.

Manner 1: The UE detects a time length for receiving, in the SCell, thedownlink synchronization channel sent by the base station; and

when determining that no downlink synchronization channel sent by thebase station is received in the SCell in a preset time length, the UEdetermines that the UE needs to perform downlink synchronization withthe base station. An example is provided for Manner 1: After receivingthe downlink synchronization channel, based on a timer or a timecounter, the UE starts or restarts the timer or resets the time counterto re-timing, so as to accordingly determine the preset time length.

Manner 2: The UE determines, according to a status of detecting andreceiving an SRS in the SCell, that the UE needs to perform downlinksynchronization with the base station. For example, if at a preset OFDMsymbol location, the UE does not detect a reference signal, or detects atime offset or a frequency offset between an OFDM symbol location of areference signal and a pre-estimated location, the UE determines thatthe UE needs to perform downlink synchronization with the base station.

Manner 3: The UE determines, according to a bit error rate or a blockerror rate during data transmission with the base station in the SCell,that the UE needs to perform downlink synchronization with the basestation. For example, if the UE determines that the bit error rate orthe block error rate during data transmission with the base station inthe SCell exceeds a preset threshold, the UE determines that the UEneeds to perform downlink synchronization with the base station.

305. The UE sends a downlink synchronization request to the basestation.

In step 305, the UE sends the downlink synchronization request to thebase station in the PCell. In a preferred manner, the downlinksynchronization request may indicate a beam range of sending thedownlink synchronization channel by the base station in the second cell.For example, step 305 may be implemented in at least one of thefollowing manners or a combination of a plurality of the followingmanners.

Manner 1: The UE sends a request message to the base station in thePCell, to instruct the base station to periodically send, by using theSCell, a beam including a downlink synchronization channel to the UE; orthe UE periodically sends indication information to the base station inthe PCell, to indicate whether the base station needs to send thedownlink synchronization channel to the UE by using the SCell.

Manner 2: After configuring a new SCell or activating the SCell, the UErequests, by using the PCell, the base station to send the downlinksynchronization channel in the SCell.

Manner 3: The UE sends, in the PCell, a request message for requestingthe base station to send the downlink synchronization channel in theSCell. Time-frequency domain resources used for sending the requestmessage by the UE may be preconfigured. For example, the UE may request,in a random access manner by using a random access preamble or aphysical random access channel (PRACH) resource different from a randomaccess purpose, the base station to send the downlink synchronizationchannel in the SCell.

Manner 4: If the UE is in an uplink channel synchronous state (an uplinktiming advance timer (TAT) is running) in the SCell, downlinksynchronization may be requested by using a downlink synchronizationSRS, physical uplink control channel (PUCCH) dedicated signaling, or amedia access control (MAC) control element (MAC CE).

306. The base station receives the downlink synchronization request ofthe UE.

That the base station determines that the UE needs to perform downlinksynchronization includes: The base station determines, according to thedownlink synchronization request, that the UE needs to perform downlinksynchronization.

307. When the base station determines that the UE needs to performdownlink synchronization in the SCell, the base station sends, accordingto location information of the UE, a beam including a downlinksynchronization channel to the UE in an SCell.

Before step 307, the location information of the UE may be firstobtained. For example, in a TDD system, the base station may performdetermining on a request message of the UE, such as a preamble or an SRSaccording to channel reciprocity, to obtain the location information ofthe UE. Certainly, the base station may also obtain the locationinformation of the UE in another manner provided in step 205 in theforegoing embodiment.

After sending the downlink synchronization request, the UE may receivethe downlink synchronization channel in the SCell according to a presetor preconfigured time window. Because different UEs have differentmoving speeds, different wireless conditions, or different UEcapabilities, the different UEs may perform downlink synchronizationwith the base station in the SCell at different time. The base stationmay configure different trigger periods for the different UEs, so thatthe different UEs detect whether the UEs need to perform synchronizationin the SCell, and request the base station to send the downlinksynchronization channel to the UEs at different moments. As shown inFIG. 5, it may be configured that among beams sent by the base stationin a period of time, only a beam that points to UE1 includes a downlinksynchronization channel.

308. The UE detects, according to the downlink synchronization referenceinformation, a downlink synchronization channel of the SCell in the beamsent by the base station.

After sending a downlink synchronization request message to the basestation, the UE waits for the downlink synchronization channel sent bythe base station. The UE may wait for the downlink synchronizationchannel according to a preset time window (for example, according to atimer or a time counter). If the UE receives no downlink synchronizationchannel in the preset time window, the UE may resend the downlinksynchronization request message, until a maximum quantity of sendingtimes is reached. If the UE detects no downlink synchronization channelin the SCell all the time, the UE reports error indication informationor resends the downlink synchronization request message to the basestation, searches another cell again, and reports a measurement resultto the base station. The UE may trigger detection on a downlinksynchronization channel of a neighboring high frequency cell andmeasurement on the high frequency cell according to a measurement resultof a neighboring low frequency cell, and try to request the downlinksynchronization channel in the measured high frequency cell. The UE mayobtain time window information according to an identifier of a beam inwhich the UE is currently located and information about a time offsetbetween a downlink synchronization channel of the beam and a downlinksynchronization channel of the PCell.

After step 308, if the UE successfully detects the downlinksynchronization channel, the UE may send indication information to thebase station at a preconfigured time-frequency resource location, tonotify the base station that the UE successfully performs downlinksynchronization in the SCell. For example, a preamble different from arandom access purpose and a synchronization request purpose may be used,or an SRS starts to be sent. A time-frequency domain resource and anencoding mode of the SRS may be different from a time-frequency domainresource and an encoding mode of an SRS used for a downlinksynchronization request purpose of the UE.

According to the downlink synchronization method provided in thisembodiment of the present disclosure, the base station sends thedownlink synchronization reference information to the UE by using thededicated signaling or the system message, and sends, in the second cellaccording to the location information of the UE, the beam including adownlink synchronization channel to the UE, so that the UE detects,according to the received downlink synchronization referenceinformation, the downlink synchronization channel in the beam sent bythe base station. The downlink synchronization reference information canbe used to instruct the UE to perform downlink synchronization in thesecond cell by referring to the downlink synchronization channel of thefirst cell, so that downlink synchronization is implemented in acellular communications system through beamforming. In addition, in thisembodiment, when determining that the UE needs to perform downlinksynchronization in the second cell, the UE may actively trigger arequest for asking the base station to send the downlink synchronizationchannel in the second cell.

In this embodiment, a frequency domain resource of a downlinksynchronization channel of the SCell may be UE-specific. That is, thebase station configures different frequency domain resources fordifferent UEs. Specifically, the base station may configure, for the UE,information about a frequency range that is in the SCell and that isoccupied by the downlink synchronization channel. When schedulinganother UE that does not need to perform synchronization in a same beam,the base station may be prevented from using a time-frequency domainresource occupied by the downlink synchronization channel.

In addition, the frequency domain resource of the downlinksynchronization channel of the SCell may be cell-specific. That is, allUEs in the SCell have the same frequency domain resource. However, atime domain resource is determined by an event or a period. All UEs inthe beam including a downlink synchronization channel may receive thedownlink synchronization channel, to obtain information about downlinksynchronization in the SCell. That is, the base station may send thesame downlink synchronization channel to all the UEs in the SCell. Inthis case, the base station determines, according to detection resultsthat are of the downlink synchronization channel and that are fed backby all the UEs, whether to configure the corresponding SCell as aserving cell of corresponding UE. Referring to FIG. 6, anotherembodiment of the present disclosure provides a downlink synchronizationmethod. The method includes the following steps.

401. Abase station sends downlink synchronization reference informationto all UEs in an SCell by using dedicated signaling or a system message.

402. All the UEs in the SCell receive the downlink synchronizationreference information sent by the base station by using the dedicatedsignaling or the system message.

403. The base station sends, according to location information of eachUE in the SCell, a beam including a downlink synchronization channel tothe UE in the SCell.

Before step 403, the location information of all the UEs may be firstobtained. The base station may traverse all beams within a sector areaof the SCell to obtain the location information of all the UEs. Forexample, the base station scans, through beam irradiation, an entirearea covered by the SCell. The beam is used to carry an indication orsignaling for obtaining the location information of each UE. For amanner of obtaining location information of one UE according to theindication or the signaling carried in the beam, refer to the manner ofobtaining the location information of the UE provided in each of theforegoing embodiments. In this way, sending the beam including adownlink synchronization channel in a direction of a beam including UEis implemented in step 403.

404. The UE detects, according to the downlink synchronization referenceinformation, a downlink synchronization channel of the SCell in the beamsent by the base station, and reports a detection result of the downlinksynchronization channel to the base station.

When sending the downlink synchronization channel, the base stationaccordingly skips a time location occupied by a skipped direction of abeam that does not include UE, or the base station readjusts, accordingto an actual quantity of beams, a time location that needs to beoccupied by each beam. Information about the quantity of beams may bebroadcast by using a PCell, so that the UE can determine, according tothe quantity of beams, information about a time offset between the UEand a downlink synchronization channel of the PCell, and an identifierof a beam in which the UE is located, a specific OFDM symbol location atwhich the UE needs to receive the synchronization channel. For example,when the quantity of beams changes, a mapping relationship among aspecific beam, a subframe index in the PCell, and a symbol location ofthe subframe index may change, or the UE may obtain time windowinformation, and receive the downlink synchronization channel in a timewindow range.

405. The base station receives the detection result that is of thedownlink synchronization channel of the SCell and that is reported bythe UE.

406. The base station determines, according to the detection result,whether to configure the SCell as a serving cell of the UE.

In this solution, the base station can determine, according to thedetection result, whether to configure the second cell as the servingcell of the UE. Therefore, the base station may send a same downlinksynchronization reference signal to all UEs in the second cell, anddetermine, according to a detection result that is of the downlinksynchronization channel of the second cell and that is reported by eachUE, whether to configure the second cell as a serving cell of the UE. Aunified downlink synchronization reference signal is designed for allthe UEs, so that signaling design overheads are reduced.

For the downlink synchronization method provided in the foregoingembodiments, an embodiment of the present disclosure provides framestructures shown in FIG. 7, FIG. 8, and FIG. 9, to describe a timeoffset between a time location of a beam of a downlink synchronizationchannel of an SCell and a time location of a beam of a downlinksynchronization channel of a PCell. As shown in FIG. 7, FIG. 8, and FIG.9, a low frequency (Low F) indicates a structure of a subframe sent inthe PCell, and a high frequency (High F) indicates a structure of asubframe sent in the SCell. A fixed time offset exists between a beam 0including a downlink synchronization channel in the SCell and a downlinksynchronization channel of the PCell. Correspondingly, a fixed timeoffset also exists between a beam 1, . . . , or a beam n in the SCelland the downlink synchronization channel of the PCell. All beams arecorresponding to different time offsets. For example, the beam 0 shownin FIG. 7 is corresponding to a symbol 0 (symbol 0) of a subframe 0 inthe PCell, and a beam 8 is corresponding to a symbol 2 (not shown in thefigure) of a subframe 1. If a fixed mapping relationship exists betweena time offset and a beam ID, beam ID information does not need to bescrambled on the downlink synchronization channel of the SCell, and theUE may obtain the beam ID information according to the time offset.Alternatively, if a fixed mapping relationship does not exist between atime offset and a beam ID, the base station may scramble the beam ID byusing the synchronization channel, so that the UE determines, accordingto the beam ID, a current symbol location (which may be updated) of theUE. The base station may determine for the SCell a downlinksynchronization channel period greater than a downlink synchronizationchannel period of the PCell, so as to reduce synchronization overheads.

That the base station sends the downlink synchronization channel in theSCell to each UE according to location information of the UE in theSCell in the foregoing embodiments may be specifically as follows: Thebase station sends, at any OFDM symbol location in a subframe in theSCell according to the location information of the UE, a beam includinga downlink synchronization channel to the UE.

Each OFDM symbol location is corresponding to one beam. All UEs in acoverage area of one beam may receive a downlink synchronization channelincluded in the beam. Two different beams are two beams whose maximumradiation directions do not overlap. It may be understood that, when oneUE in the SCell performs downlink synchronization, a downlinksynchronization channel for the UE may be sent in a beam that iscorresponding to location information of the UE and that is at any OFDMsymbol location in a subframe; or when all UEs in the SCell performdownlink synchronization, a same downlink synchronization channel issent to all the UEs, and the downlink synchronization channel of each UEmay be sent in a beam that is corresponding to location information ofthe UE and that is at an OFDM symbol location in a subframe.

As shown in FIG. 7, different beams including a downlink synchronizationchannel are separately sent at different OFDM symbol locations in onesubframe in the SCell. These beams have different directions, so that aplurality of UE locations can be covered. If a total quantity of beamsis greater than a maximum quantity of beams that can be sent in onesubframe (in consideration of a quantity of symbols and a beam switchingtime that are occupied by a synchronization channel beam), differentbeams may be sent in a plurality of subframes. A PSS and an SSS may beredesigned as an evolved synchronization signal (eSS) in one symbol, tosimplify a synchronization process.

Further, each OFDM symbol includes beams in at least two directions. Allthe beams in the at least two directions use different radio frequencychains RF chains. For example, as shown in FIG. 8, different beamsincluding a downlink synchronization channel are separately sent atdifferent OFDM symbol locations in one subframe in the SCell. However, aplurality of different beams may be simultaneously sent at a same OFDMsymbol location by using a plurality of RF chains. As shown in FIG. 8,abeam 0 and a beam n are simultaneously sent at a location of a symbol 0by respectively using one RF chain. Therefore, more beams may be sent byusing fewer OFDM symbols, so as to cover more UEs more quickly. In thiscase, a same time offset between a time location of a downlinksynchronization channel of the SCell and a time location of a downlinksynchronization channel of the PCell may be corresponding to a pluralityof beam IDs.

Optionally, step 403 may be specifically as follows: The base stationsends, at a fixed OFDM symbol location in any subframe in the SCellaccording to location information of the UE, different beams including adownlink synchronization channel to the UE. It may be understood that,when one UE in the SCell performs downlink synchronization, a downlinksynchronization channel for the UE may be sent in a beam that iscorresponding to location information of the UE and that is at a fixedOFDM symbol location in any subframe; or when all UEs in the SCellperform downlink synchronization, a same downlink synchronizationchannel is sent to all the UEs, and downlink synchronization channels ofall the UEs may be separately sent in beams that are corresponding tolocation information of all the UEs and that are at fixed OFDM symbollocations in different subframes. For example, as shown in FIG. 9, onebeam including a synchronization channel is sent at a specified OFDMsymbol location in each subframe in the SCell. Therefore, a plurality ofdifferent beams need to be sent in a plurality of subframes (a beam 0, abeam 1, a beam 2, . . . ). Further, each OFDM symbol includes beams inat least two directions. All the beams in the at least two directionsuse different radio frequency chains RF chains. This solution is similarto that in FIG. 8. Details are not described herein.

Referring to FIG. 10, an embodiment of the present disclosure providesan apparatus, configured to perform the downlink synchronization methodprovided in the foregoing embodiments. The apparatus may be a basestation, and may include:

a sending unit 11, configured to send downlink synchronization referenceinformation to UE by using dedicated signaling or a system message,where the downlink synchronization reference information is used toinstruct the UE to perform downlink synchronization in a second cell byreferring to a downlink synchronization channel of a first cell.

The sending unit 11 is further configured to send, in the second cellaccording to location information of the UE, a beam including a downlinksynchronization channel to the UE, where the first cell and the secondcell include an overlapped coverage area.

According to the downlink synchronization apparatus provided in thissolution, the base station sends the downlink synchronization referenceinformation to the UE by using the dedicated signaling or the systemmessage, and sends, in the second cell according to the locationinformation of the UE, the beam including a downlink synchronizationchannel to the UE, so that the UE detects, according to the receiveddownlink synchronization reference information, the downlinksynchronization channel in the beam sent by the base station. Thedownlink synchronization reference information can be used to instructthe UE to perform downlink synchronization in the second cell byreferring to the downlink synchronization channel of the first cell, sothat downlink synchronization is implemented in a cellularcommunications system through beamforming.

Optionally, referring to FIG. 10, the base station further includes:

a processing unit 12, configured to determine that the UE needs toperform downlink synchronization in the second cell.

Specifically, the processing unit 12 is configured to detect a timelength for sending the downlink synchronization channel in the secondcell to the UE; and

when determining that no downlink synchronization channel is sent in thesecond cell to the UE in a preset time length, the processing unit 12determines that the UE needs to perform downlink synchronization; or

the processing unit 12 is configured to determine, according to a statusof detecting and receiving a sounding reference signal SRS of the UE inthe second cell, that the UE needs to perform downlink synchronization;or

the processing unit 12 is configured to determine, according to a biterror rate or a block error rate during data transmission in the secondcell with the UE, that the UE needs to perform downlink synchronization.

In this solution, the base station may actively trigger sending of thedownlink synchronization channel when determining that the UE needs toperform downlink synchronization in the second cell.

Alternatively, the base station further includes a receiving unit 13,configured to receive a downlink synchronization request of the UE.

The processing unit 12 is configured to determine, by the base stationaccording to the downlink synchronization request, that the UE needs toperform downlink synchronization.

In this embodiment, when determining that the UE needs to performdownlink synchronization in the second cell, the UE may actively triggera request for asking the base station to send the downlinksynchronization channel in the second cell.

Referring to FIG. 10, the base station further includes:

a configuration unit 14, configured to configure carrier aggregation forthe UE, where on an aggregated carrier, the first cell is on a carrierwhose frequency domain is located on a first frequency band, the secondcell is on a carrier whose frequency domain is located on a secondfrequency band, the first frequency band is a relatively low frequencyband, and the second frequency band is a relatively high frequency band.

Optionally, the receiving unit 13 is configured to receive a detectionresult that is of a downlink synchronization channel of the second celland that is reported by the UE.

The processing unit 12 is configured to determine, according to thedetection result received by the receiving unit, whether to configurethe second cell as a serving cell of the UE.

In this solution, the base station can determine, according to thedetection result, whether to configure the second cell as the servingcell of the UE. Therefore, the base station may send a same downlinksynchronization reference signal to all UEs in the second cell, anddetermine, according to a detection result that is of the downlinksynchronization channel of the second cell and that is reported by eachUE, whether to configure the second cell as a serving cell of the UE. Aunified downlink synchronization reference signal is designed for allthe UEs, so that signaling design overheads are reduced.

Optionally, the sending unit 11 is specifically configured to send, atany OFDM symbol location in a subframe in the second cell according tothe location information, the beam including a downlink synchronizationchannel to the UE.

Optionally, the sending unit 11 is specifically configured to send, at afixed OFDM symbol location in any subframe in the second cell accordingto the location information, different beams including a downlinksynchronization channel to the UE.

Optionally, each OFDM symbol includes beams in at least two directions.All the beams in the at least two directions use different radiofrequency chains RF chains.

In addition, a fixed time offset is used between a time location of abeam of the downlink synchronization channel of the second cell and atime location of a beam of the downlink synchronization channel of thefirst cell. The downlink synchronization reference information mayfurther include but is not limited to at least one of the followinginformation: a mapping relationship between the time offset and a beamID of the downlink synchronization channel of the second cell, afrequency band of the second cell, bandwidth of the second cell, a PCIof the second cell, a frequency range in which the downlinksynchronization channel of the second cell is located, a quantity ofbeams of the second cell, or a beam width of the second cell.

It should be noted that the sending unit 11 in this embodiment may be aninterface circuit that has a sending function on the base station, suchas a transmitter or an information sending interface. The receiving unit13 may be an interface circuit that has a receiving function on the basestation, such as a receiver or an information receiving interface. Theprocessing unit 12 and the configuration unit 14 may be separatelydisposed processors, or may be integrated into one processor of the basestation for implementation. In addition, the processing unit 12 and theconfiguration unit 14 may be stored in a memory of the base station in aform of program code. Functions of the processing unit 12 and theconfiguration unit 14 are invoked and executed by a processor of thebase station. The processor described herein may be a central processingunit (CPU), or an application-specific integrated circuit (ASIC), or maybe one or more integrated circuits configured to implement thisembodiment of the present disclosure.

Referring to FIG. 11, an embodiment of the present disclosure providesan apparatus. The apparatus may be UE, and includes:

a receiving unit 21, configured to receive downlink synchronizationreference information sent by a base station by using dedicatedsignaling or a system message, where the downlink synchronizationreference information is used to instruct the UE to perform downlinksynchronization in a second cell by referring to a downlinksynchronization channel of a first cell; and

a detection unit 22, configured to detect, according to the downlinksynchronization reference information received by the receiving unit 21,a downlink synchronization channel of the second cell in a beam sent bythe base station, where the first cell and the second cell include anoverlapped coverage area.

In this solution, the base station sends the downlink synchronizationreference information to the UE by using the dedicated signaling or thesystem message, and sends, in the second cell according to locationinformation of the UE, a beam including a downlink synchronizationchannel to the UE, so that the UE detects the downlink synchronizationchannel according to the received downlink synchronization referenceinformation. The downlink synchronization reference information can beused to instruct the UE to perform downlink synchronization in thesecond cell by referring to the downlink synchronization channel of thefirst cell, so that downlink synchronization is implemented in acellular communications system through beamforming.

Optionally, referring to FIG. 11, the UE further includes:

a sending unit 23, configured to report a detection result of thedownlink synchronization channel of the second cell to the base station.

Optionally, the UE further includes:

a processing unit 24, configured to determine that the UE needs toperform downlink synchronization with the base station; and

a sending unit 23, configured to send a downlink synchronization requestto the base station.

In this solution, the base station may actively trigger sending of thedownlink synchronization channel when determining that the UE needs toperform downlink synchronization in the second cell.

Optionally, the processing unit 24 is specifically configured to detecta time length for receiving, in the second cell, the downlinksynchronization channel sent by the base station; and

when determining that no downlink synchronization channel sent by thebase station is received in the second cell in a preset time length, theprocessing unit 24 determines that the UE needs to perform downlinksynchronization with the base station; or

the processing unit 24 determines, according to a status of detectingand receiving an SRS in the second cell, that the UE needs to performdownlink synchronization with the base station; or

the processing unit 24 determines, according to a bit error rate or ablock error rate during data transmission in the second cell with thebase station, that the UE needs to perform downlink synchronization withthe base station.

In this solution, when determining that the UE needs to perform downlinksynchronization in the second cell, the UE may actively trigger arequest for asking the base station to send the downlink synchronizationchannel in the second cell.

Optionally, the UE further includes:

a sending unit 23, configured to report a detection result of thedownlink synchronization channel to the base station.

Optionally, the detection unit 22 is specifically configured to detect,according to information about the downlink synchronization channel ofthe first cell, the downlink synchronization channel of the second cellin the beam sent by the base station.

It should be noted that the sending unit 23 in this embodiment may be aninterface circuit that has a sending function on the UE, such as atransmitter or an information sending interface. The receiving unit 21may be an interface circuit that has a receiving function on the UE,such as a receiver or an information receiving interface. The detectionunit 22 and the processing unit 24 may be separately disposedprocessors, or may be integrated into one processor of the UE forimplementation. In addition, the detection unit 22 and the processingunit 24 may be stored in a memory of the UE in a form of program code.Functions of the detection unit 22 and the processing unit 24 areinvoked and executed by a processor of the UE. The processor describedherein may be a central processing unit (CPU), or anapplication-specific integrated circuit (ASIC), or may be one or moreintegrated circuits configured to implement this embodiment of thepresent disclosure.

Referring to FIG. 12, an embodiment of the present disclosure providesan apparatus, configured to perform the foregoing downlinksynchronization method. The apparatus may be a base station, and mayinclude a processor 31, a first interface circuit 32, a second interfacecircuit 33, a memory 34, and a bus 35. The processor 31, the firstinterface circuit 32, the second interface circuit 33, and the memory 34are connected to each other and communicate with each other by using thebus 35.

It should be noted that the processor 31 herein may be one processor, ormay be a general term of a plurality of processing elements. Forexample, the processor may be a central processing unit (CPU), or may bean application-specific integrated circuit (ASIC), or may be one or moreintegrated circuits configured to implement this embodiment of thepresent disclosure, such as one or more microprocessors (e.g., a digitalsignal processor (DSP)), or one or more field programmable gate arrays(FPGA).

The memory 34 may be one storage apparatus or may be a general term of aplurality of storage elements, and is configured to store executableprogram code or a parameter, data, and the like that are required forrunning an access network management device. In addition, the memory 34may include a random access memory (RAM), or may include a nonvolatilememory (NVRAM), such as a magnetic disk storage or a flash (Flash)memory.

The bus 35 may be an industry standard architecture (ISA) bus, aperipheral component interconnect (PCI) bus, an extended industrystandard architecture (EISA) bus, or the like. The bus 35 may beclassified into an address bus, a data bus, a control bus, and the like.For ease of representation, only one bold line is used in FIG. 12, butit does not indicate that there is only one bus or one type of bus.

The processor 31 is configured to execute a program in the memory, toperform the method provided in the foregoing method embodiment withreference to the first interface circuit 32 and the second interfacecircuit 33.

Specifically, the processor 31 is configured to execute the program inthe memory to execute functions of the processing unit and theconfiguration unit in the base station in the foregoing embodiment.

The first interface circuit 32 is configured to execute a function ofthe sending unit in the base station in the foregoing embodiment.

The second interface circuit 33 is configured to execute a function ofthe receiving unit in the base station in the foregoing embodiment.

Referring to FIG. 13, an embodiment of the present disclosure providesan apparatus, configured to perform the foregoing downlinksynchronization method. The apparatus may be UE, and may include aprocessor 41, a first interface circuit 42, a second interface circuit43, a memory 44, and a bus 45. The processor 41, the first interfacecircuit 42, the second interface circuit 43, and the memory 44 areconnected to each other and communicate with each other by using the bus45.

It should be noted that the processor 41 herein may be one processor, ormay be a general term of a plurality of processing elements. Forexample, the processor may be a central processing unit CPU, or may bean application-specific integrated circuit ASIC, or may be one or moreintegrated circuits configured to implement this embodiment of thepresent disclosure, such as one or more microprocessors (e.g., a digitalsignal processor (DSP)), or one or more field programmable gate arrays(FPGA).

The memory 44 may be one storage apparatus or may be a general term of aplurality of storage elements, and is configured to store executableprogram code or a parameter, data, and the like that are required forrunning an access network management device. In addition, the memory 44may include a random access memory (RAM), or may include a nonvolatilememory (NVRAM), such as a magnetic disk storage or a flash (Flash)memory.

The bus 45 may be an industry standard architecture (ISA) bus, aperipheral component interconnect (PCI) bus, an extended industrystandard architecture (EISA) bus, or the like. The bus 45 may beclassified into an address bus, a data bus, a control bus, and the like.For ease of representation, only one bold line is used in FIG. 13, butit does not indicate that there is only one bus or one type of bus.

The processor 41 is configured to execute a program in the memory, toperform the method provided in the foregoing method embodiment withreference to the first interface circuit 42 and the second interfacecircuit 43.

Specifically, the processor 41 is configured to execute the program inthe memory to execute functions of the detection unit and the processingunit in the UE in the foregoing embodiment.

The first interface circuit 42 is configured to execute a function ofthe receiving unit in the UE in the foregoing embodiment.

The second interface circuit 43 is configured to execute a function ofthe sending unit in the UE in the foregoing embodiment.

It should be understood that sequence numbers of the foregoing processesdo not mean execution sequences in various embodiments of the presentdisclosure. The execution sequences of the processes should bedetermined according to functions and internal logic of the processes,and should not be construed as any limitation on the implementationprocesses of the embodiments of the present disclosure.

A person of ordinary skill in the art may be aware that, in combinationwith the examples described in the embodiments disclosed in thisspecification, units and algorithm steps may be implemented byelectronic hardware or a combination of computer software and electronichardware. Whether the functions are performed by hardware or softwaredepends on particular applications and design constraint conditions ofthe technical solutions. A person skilled in the art may use differentmethods to implement the described functions for each particularapplication, but it should not be considered that the implementationgoes beyond the scope of the present disclosure.

It may be clearly understood by a person skilled in the art that, forthe purpose of convenient and brief description, for a detailed workingprocess of the foregoing system, apparatus, and unit, reference may bemade to a corresponding process in the foregoing method embodiments, anddetails are not described herein again.

In the several embodiments provided in this application, it should beunderstood that the disclosed system, device, and method may beimplemented in other manners. For example, the described deviceembodiment is merely an example. For example, the unit division ismerely logical function division and may be other division in actualimplementation. For example, a plurality of units or components may becombined or integrated into another system, or some features may beignored or not performed. In addition, the displayed or discussed mutualcouplings or direct couplings or communication connections may beimplemented through some interfaces. The indirect couplings orcommunication connections between the devices or units may beimplemented in electronic, mechanical, or other forms.

The units described as separate parts may or may not be physicallyseparate, and parts displayed as units may or may not be physical units,may be located in one position, or may be distributed on a plurality ofnetwork units. Some or all of the units may be selected according toactual requirements to achieve the objectives of the solutions of theembodiments.

In addition, functional units in the embodiments of the presentdisclosure may be integrated into one processing unit, or each of theunits may exist alone physically, or two or more units are integratedinto one unit.

When the functions are implemented in the form of a software functionalunit and sold or used as an independent product, the functions may bestored in a computer-readable storage medium. Based on such anunderstanding, the technical solutions of the present disclosureessentially, or the part contributing to the prior art, or some of thetechnical solutions may be implemented in a form of a software product.The computer software product is stored in a storage medium, andincludes several instructions for instructing a computer device (whichmay be a personal computer, a server, a network device, or the like) toperform all or some of the steps of the methods described in theembodiments of the present disclosure. The foregoing storage mediumincludes: any medium that can store program code, such as a USB flashdrive, a removable hard disk, a read-only memory (ROM), a random accessmemory (RAM), a magnetic disk, or an optical disc.

The foregoing descriptions are merely specific implementations of thepresent disclosure, but are not intended to limit the protection scopeof the present disclosure. Any variation or replacement readily figuredout by a person skilled in the art within the technical scope disclosedin the present disclosure shall fall within the protection scope of thepresent disclosure. Therefore, the protection scope of the presentdisclosure shall be subject to the protection scope of the claims.

What is claimed is:
 1. A communications method, comprising: configuring,by a first base station, carrier aggregation or dual connection on afirst cell and a second cell for a terminal; sending, by the first basestation, downlink synchronization reference information to the terminalby using dedicated signaling or a system message, the downlinksynchronization reference information for instructing the terminal toperform downlink synchronization in the second cell by applying arelationship between a time domain source of a second downlinksynchronization channel of the second cell and a time domain source of afirst downlink synchronization channel of the first cell, wherein thesecond cell is on a second carrier and the first cell is on a firstcarrier, wherein a second downlink synchronization channel of the secondcell is detected by the terminal according to the downlinksynchronization reference information.
 2. The method according to claim1, further comprising: sending, by a second base station and in thesecond cell, the second downlink synchronization channel to theterminal, wherein the second base station is the first base station or abase station other than the first base station.
 3. The method accordingto claim 2, wherein the method further comprises: determining, by thefirst base station, that the terminal needs to perform downlinksynchronization in the second cell.
 4. The method according to claim 2,wherein after sending, by the second base station in the second cell,the second downlink synchronization channel to the terminal, the methodfurther comprises: receiving, by the first base station, a detectionresult of the second downlink synchronization channel of the second cellthat is reported from the terminal; and determining, by the first basestation, according to the detection result, whether to configure thesecond cell as a serving cell of the terminal.
 5. The method accordingto claim 2, wherein sending, by the second base station in the secondcell, the second downlink synchronization channel to the terminalcomprises: sending, by the second base station at any orthogonalfrequency division multiplexing (OFDM) symbol location in a subframe inthe second cell, the second downlink synchronization channel to theterminal.
 6. The method according to claim 5, wherein each OFDM symbolcomprises beams in at least two directions, and all the beams in the atleast two directions use different radio frequency (RF) chains.
 7. Themethod according to claim 2, wherein sending, by the second base stationin the second cell, the second downlink synchronization channel to theterminal comprises: sending, by the second base station at a fixedorthogonal frequency division multiplexing (OFDM) symbol location in asubframe in the second cell, the second downlink synchronization channelto the terminal.
 8. The method according to claim 1, wherein the secondcell is a neighbor cell of the first cell.
 9. A communications method,comprising: receiving, by a terminal, downlink synchronization referenceinformation from a base station by using dedicated signaling or a systemmessage, wherein the downlink synchronization reference informationinstructs the terminal to perform downlink synchronization in a secondcell by applying a relationship between a time domain source of a seconddownlink synchronization channel of the second cell and a time domainsource of a first downlink synchronization channel of the first cell,wherein the second cell is on a second carrier and the first cell is ona first carrier; and detecting, by the terminal according to thedownlink synchronization reference information, a second downlinksynchronization channel of the second cell.
 10. The method according toclaim 9, wherein after detecting, by the terminal according to thedownlink synchronization reference information, the second downlinksynchronization channel of the second cell, the method furthercomprises: reporting, by the terminal, a detection result of the seconddownlink synchronization channel of the second cell.
 11. The methodaccording to claim 9, wherein detecting, by the terminal according tothe downlink synchronization reference information, the second downlinksynchronization channel of the second cell comprises: detecting, by theterminal according to information about the first downlinksynchronization channel of the first cell, the second downlinksynchronization channel of the second cell.
 12. The method according toclaim 9, further comprising: receiving, by the terminal, a configurationabout carrier aggregation or dual connection on the first cell and thesecond cell for the terminal.
 13. The method according to claim 9,wherein the second cell is a neighbor cell of the first cell.
 14. Anapparatus, comprising: a receiving circuit to receive downlinksynchronization reference information from a base station by usingdedicated signaling or a system message, wherein the downlinksynchronization reference information instructs the apparatus to performdownlink synchronization in a second cell by applying a relationshipbetween a time domain source of a second downlink synchronizationchannel of the second cell and a time domain source of a first downlinksynchronization channel of the first cell, wherein second cell is on asecond carrier and the first cell is on a first carrier; and a processorto detect, according to the downlink synchronization referenceinformation received by the receiving circuit, a second downlinksynchronization channel of the second cell.
 15. The apparatus accordingto claim 14, further comprising: a transmitting circuit to report adetection result of the second downlink synchronization channel of thesecond cell.
 16. The apparatus according to claim 14, wherein theprocessor is to detect, according to information about the firstdownlink synchronization channel of the first cell, the second downlinksynchronization channel of the second cell.
 17. The apparatus accordingto claim 14, wherein the receiving circuit is further configured toreceive a configuration about carrier aggregation or dual connection onthe first cell and the second cell for the terminal.
 18. The apparatusaccording to claim 14, wherein the second cell is a neighbor cell of thefirst cell.